This invention relates to a linear positioning mechanism, and more particularly to a positioning mechanism for use with an overbed table such as is used in a hospital, nursing home or the like.
An overbed table typically has a wheeled base supporting a lower pedestal column to which an upper column is movably mounted. The upper column is typically connected at its upper end to the underside of the overbed table. The base is wheeled under the bed, with the table extending over a patient lying or sitting on the bed. The upper column is movable on the lower column for adjusting the vertical height of the table, and a latching mechanism including a manually operable lever is provided for maintaining the table at a desired elevation. As a safety feature, the latching mechanism is designed to allow upward movement of the table top without operation of the manually operable lever.
Various mechanisms are known for providing vertical positioning of an overbed table. One such mechanism is disclosed in U.S. Pat. No. 4,601,246, owned by the assignee of the present invention. The structure shown in this patent provides a lower outer column in which the upper column is telescopically mounted. A shelf is formed by the upper end of the lower column, with a gap between the upper and lower columns. With this construction, it is possible for spilled liquids to flow through the gap between the upper and lower columns and into the interior of the lower column. The presence of liquids in the interior of the lower column can result in problems in operation of the assembly, and it is often difficult to clean the inside of the lower column to remove such contaminants. This problem is one which has been recognized by those in the health care field. A solution to this problem is provided by installing an upper outer sleeve concentric with the upper column to prevent entry of liquids into the interior of the lower column.
A structure which avoids the above-noted problem is shown in U.S. Pat. No. 3,890,907. A drawback to the construction shown in this patent, however, is that it incorporates a rather complicated positioning mechanism including cables, pulleys, drums, cable retainers, shafts, springs and other structural elements.
It is an object of the present invention to provide a relatively simple linear positioning mechanism for selectively fixing the position of a first member relative to a second member. It is a further object of the invention to provide a support mechanism for an overbed table which incorporates such a linear positioning mechanism, and which employs a design incorporating an upper outer column so as to prevent contamination of the interior of the support. In accordance with the invention, a linear positioning apparatus for selectively fixing the position of a first member, such as an overbed table, relative to a second member, such as a base, includes an elongated threaded member interconnected with one of the first or second members, and a threaded nut engageable with the threaded member and interconnected with the other of the first or second members. The elongated threaded member may comprise a high lead screw. The lead screw and the nut are interconnected so that movement of the overbed table relative to the pedestal base causes relative movement between the lead screw and the nut. A selectively engageable clutch means is associated with the apparatus for selectively allowing and preventing relative movement of the nut and lead screw, and thereby selectively allowing and preventing relative movement between the overbed table and the base. In one embodiment, the lead screw is interconnected with the overbed table, and its position is fixed relative to the upper column. The threaded nut is fixed to the upper end of the lower column so that rotation of the nut is prevented. Upward and downward movement of the overbed table relative to the base causes rotation of the lead screw as it moves through the threaded nut. The clutch mechanism is engageable with the lead screw for selectively preventing and allowing its rotation in response to a downward force applied to the table. When the clutch mechanism is released, a downward force on the table causes the table to move relative to the base, and the lead screw to rotate within the nut. When the clutch mechanism is engaged, so that rotation of the lead screw in one direction is prevented, the overbed table may not be moved downwardly relative to the base.
In one embodiment, the clutch mechanism is a unidirectional spring clutch in which a first clutch member is affixed to the upper column, and a second clutch member is mounted to the lead screw. A spring is wrapped around the first and second clutch members, and is biased towards a normal position in which it couples the first and second clutch members together by engaging the outer surfaces of each clutch member. A release mechanism is provided for selectively moving the spring out of engagement with the first and second clutch members so as to allow relative movement therebetween. With the described clutch mechanism, it is not necessary to operate the release mechanism in order to effect upward movement of the table. When it is desired to lower the table, the release mechanism is actuated so as to move the clutch spring out of engagement with the first and second clutch members and a downward force is applied to the table, resulting in rotation of the lead screw within the nut. Upon release of the release mechanism, the spring returns to its normal condition so as to engage the outer surfaces of the first and second clutch members and thereby prevent rotation of the lead screw, thereby fixing the table at the desired elevation.
The invention provides a simple and efficient design for a linear positioning mechanism for an assembly such as an overbed table. The apparatus of the invention is easily constructed, and provides a quiet and easily operated support for an overbed table.